Unit #5: The Lipids: Triglycerides, Phospholipids, and Sterols
In-Class Activity
Saturated vs. Unsaturated Fatty Acids
Purpose:
The purpose of this activity is to provide students with a visual representation of
a saturated fatty acid versus an unsaturated fatty acid to help students distinguish
between the types of fatty acids and understand what happens when hydrogenation of
an unsaturated fatty acid occurs.
Background Information:
A fatty acid is an organic acid—a chain of atoms with hydrogens attached—that
has an acid group (COOH) at one end and a methyl group (CH3) at the other end.
There are three factors that chemist’s use to differentiate between the types of fatty
acids: 1) the length of the carbon chain; 2) the degree of unsaturation; and 3) the
location of the double bonds.
1) Most naturally occurring fatty acids contain even numbers of carbons in their
chains—up to 24 carbons in length. The long-chain (12-24 carbons) fatty acids of
meat, fish, and vegetable oils are most common in the diet. Smaller amounts of
medium-chain (6-10 carbons) and short-chain (fewer than 6 carbons) fatty acids also
occur, primarily in dairy products.
2) A saturated fatty acid is fully loaded with hydrogen atoms and contains only
single bonds between its carbon atoms. The double bond is the point of saturation, the
double bond of a fatty acid, where hydrogen atoms can easily be added to the
structure. With two hydrogens missing and a double bond—is an unsaturated fatty
acid, a fatty acid that lacks hydrogen atoms and has at least one double bond between
carbons. A polyunsaturated fatty acid has two or more carbon-to-carbon double bonds.
Polyunsaturated fatty acid that lacks four or more hydrogen atoms and has two or more
double bonds between carbons.
3) Fatty acids differ not only in the length of their chains and their degree of
saturation, but also in the locations of their double bonds. Chemists identify
polyunsaturated fatty acids by the position of the double bond nearest the methyl end
(CH3) end of the carbon chain, which is described by an omega number. A
polyunsaturated fatty acid with its first double bond three carbons away from the
methyl end is an omega-3 fatty acid, Similarly, an omega-6 fatty acid is a
polyunsaturated fatty acid with its first double bond six carbons away from the methyl
end.
Directions:
Complete the following fatty acid chains below by doing the following:
1) Add hydrogen atoms to all the carbons in the fatty acid chains by recording
“H” in red ink.
2) Add an oxygen atom (using blue ink) to the “acid end” of the fatty acid chain
utilizing a double bond.
3) Highlight the “methyl end” in green.
4) Highlight the “acid end” in orange.
5) Add double bond(s) where the degree of unsaturation is located in your
chains.
Fatty Acid Chain #1: Saturated Fatty Acid (Stearic Acid):
H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-OH
Fatty Acid Chain #2: Monounsaturated Fatty Acid (Oleic Acid):
H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-OH
Fatty Acid Chain #3: Polyunsaturated Fatty Acid (Linoleic Acid):
H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-OH
Fatty Acid Chain #4: Omega-3 Fatty Acid:
H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-OH
Fatty Acid Chain #5: Omega-6 Fatty Acid:
H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-OH